Long-term Debt - Principal and Interest Payments of Debt (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Principal
Total scheduled payments, Principal, 2019	$ 5,333
Total scheduled payments, Principal, 2020	5,333
Total scheduled payments, Principal, 2021	4,889
Total scheduled payments, Principal, Thereafter	0
Total scheduled payments, Principal	15,555
Less: debt discount and issuance costs	(1,476)
Less: current portion of long-term debt	(4,812)	$ (444)
Long-term debt	9,267
Interest
Total scheduled payments, Interest, 2019	1,061
Total scheduled payments, Interest, 2020	633
Total scheduled payments, Interest, 2021	198
Total scheduled payments, Interest, Thereafter	0
Total scheduled payments, Interest	1,892
Back-end fee
Total scheduled payments, Back-end fee, 2019	0
Total scheduled payments, Back-end fee, 2020	0
Total scheduled payments, Back-end fee, 2021	1,440
Total scheduled payments, Back-end fee, Thereafter	0
Total scheduled payments, Back-end fee	1,440
Total scheduled payments, 2019	6,394
Total scheduled payments, 2020	5,966
Total scheduled payments, 2021	6,527
Total scheduled payments, Thereafter	0
Total scheduled payments	$ 18,887